In recent years, disc recording and reproducing apparatuses have been compatible with a variety of recording media including those for DVD (Digital Versatile Disc), BD (Blue-ray Disc), CD (Compact Disc), personal computers and so on and have the applications thereof diversified year by year. In addition, in accordance with demands for increasing the densities of the recording media, the disc recording and reproducing apparatuses are demanded to have high performances and high recording densities. In particular, disc recording and reproducing apparatuses for laptop type personal computers and mobile uses are also eagerly demanded to have reductions in size and thickness.
FIG. 26 is a schematic view of an optical system showing an essential part of an optical head device of a prior art. The optical head device of the prior art has objective lenses 7 and 14 that condense laser lights of different wavelengths emitted from light sources, and radiate the lights to an information recording medium 8, and upward-reflecting mirrors 5 and 12 for reflecting the laser lights to change their directions. Referring to FIG. 26, it is assumed that the thickness of the objective lenses 7 and 14 is A, the movable range of the objective lenses 7 and 14 is B, the luminous flux diameter of the laser light incident on the upward-reflecting mirrors 5 and 12 is C, dimensional margins in consideration for the chipped edge's, non-uniform deposition of reflection coatings and so on of the upward-reflecting mirrors 5 and 12 is D, a distance (working distance) from the objective lenses 7 and 14 to the information recording medium 8 is Dw, the total height of the optical head device is H, the numerical aperture of the objective lenses is NA, and the focal distance is f. In this case, the luminous flux diameter C is expressed by the following Equation (1), and the total height H of the optical head device is expressed by the following Equation (2):C=2×NA×f  (1); andH=A+B+C+D+Dw  (2).
FIG. 27 is a schematic view of an optical system showing an essential part of the optical head device in a case where laser light having a wavelength of 405 nm is used in the optical head device shown in FIG. 26. Referring to FIG. 27, the laser light having a wavelength of 405 nm emitted from a light source is transformed from a linearly polarized light into a circularly polarized light by a quarter-wave plate 6 and made incident on the upward-reflecting mirror 5. The upward-reflecting mirror 5 transmits the laser light having a wavelength of 405 nm. The laser light that has been transmitted through the upward-reflecting mirror 5 is made incident on the upward-reflecting mirror 12 and reflected by the upward-reflecting mirror 12. The laser light reflected by the upward-reflecting mirror 12 is condensed by the objective lens 14, and this leads to formation of a light spot on the information recording medium 8. At this time, the objective lens 14 moves to a position 14a at maximum in a focus direction within a movable range B in order to follow the surface sway of the information recording medium 8, and the objective lens 7 moves to a position 7a at maximum interlockingly with the objective lens 14.
FIG. 28 is a schematic view of an optical system showing an essential part of the optical head device in a case where laser light having a wavelength of 650 nm is used in the optical head device shown in FIG. 26. Referring to FIG. 28, the laser light having a wavelength of 650 nm emitted from a light source is transformed from a linearly polarized light into a circularly polarized light by the quarter-wave plate 6 and reflected by the upward-reflecting mirror 5. The laser light reflected by the upward-reflecting mirror 5 is condensed by the objective lens 7, and this leads to formation of a light spot on the information recording medium 8. At this time, the objective lens 7 moves to a position 7b at maximum in the focus direction within the movable range B in order to follow the surface sway of the information recording medium 8, and the objective lens 14 moves to a position 14b at maximum interlockingly with the objective lens 7.                See Patent Document 1: JP-11-120587-A.        
However, since the objective lenses 7 and 14 interlockingly move in the optical head device of the prior art when the objective lenses 7 and 14 have mutually different thicknesses, there is a possibility that the upward-reflecting mirror 12 and the objective lens 14 collide with each other when the objective lens 7 moves in the focus direction as shown in FIG. 28. The collision possibly occurs similarly when the objective lenses 7 and 14 have different working distances Dw or when the placement position of one of the objective lenses 7 and 14 is located on the side lower than the other one. Accordingly, there has been such a problem that the height H of the optical head device has needed to be increased in order to avoid the collision, and it has been difficult to reduce the size and thickness of the optical head device.
An object of the present invention is to solve the above problems and to provide a compact thin optical head device as well as a disc recording apparatus, a disc reproducing apparatus and a disc recording and reproducing apparatus each provided with the same optical device.